The object of the present invention is a floater dryer for drying a web-like material, particularly a paper or cardboard web, said dryer comprising a plurality of radiation/air blowing units which are provided on one or both sides of the web, and said units being designed to be blow boxes of which the side facing the web is a contact-free carrier surface and in conjunction with which is blown an air jet, or air jets, through a nozzle aperture opening onto the leading or trailing edge of said carrier surface. The jets will have a component of substantial magnitude parallel to the plane of said carrier surface.
In addition, the invention concerns a procedure for enhancing the drying of web-like moving material wherein combined radiation and air drying is applied, this being implemented with combined radiation/air blowing units, through their nozzle slit or slits an air blow jet or air blow jets being directed into a treatment interval and the web that is being dried being supported with their aid without contact.
In the prior art, so-called floater dryers are known in which a paper web, a cardboard web or equivalent is dried without contact. Floater dryers are for instance used in paper coating apparatus after a roll or brush applicator to support without contact and to dry the web which is wet owing to the coating substance. Various drying and supporting air nozzles and arrays thereof are applied in floater dryers. Said blow nozzles may be classified by two groups: over-pressure nozzles and subatmospheric pressure nozzles, both kinds being applicable in the floater dryer and procedure of the invention.
The commonest floater dryers of prior art in present use are exclusively based on air blowings. Partly for this reason, the floater dryers become rather bulky because the distance over which the floater dryer is active has to be rather long so that high enough drying effect might be achieved. In part, these drawbacks are due to the fact that in air drying the penetration depth of drying is rather minimal.
In prior art various types of dryers are known which are based on the effect of radiation, above all of infrared radiation. Using infrared radiation affords the advantage that the radiation has a fairly high penetration depth, which increases with decreasing wavelength. Application of infrared dryers in drying a paper web has been hampered, among other things, by fire hazard because infrared radiators attain rather high temperatures, e.g. 2000.degree. C. if it is desired to achieve a drying radiation with sufficiently short wavelength.
Regarding the state of art, reference is furthermore made to DE OS No. 2351280, which discloses a certain kind of combination of floater dryer and infrared dryer operating with over-pressure nozzles. In the patent application just cited is disclosed a one-sided floater dryer comprising consecutive nozzle boxes spaced in relation to each other. These boxes have nozzle slits on their marginal parts, through said slits air jets being directed against the web thereabove, specifically at right angles, these jets when they meet the web being deflected outward at the nozzle box. Between said nozzles infrared radiators have been disposed which fill the interval between nozzles. As far as the present applicant is aware, said dryer has not come into any widespread use, at least, which is believed to be due to the circumstance that it has not been understood, neither structurally nor in the energy economy respect, in said nozzle design to combine air and radiation drying in an advantageous way. The structure is moreover one-sided, and it requires rather much space in the direction of travel of the web if one wishes to attain high enough drying power, for instance in paper after-treatment installations.
A drawback with major effect encumbering said DE-OS, and other infra dryers of prior art, is that in them the space between the infrared dryer and the web being dried is not ventilated, with the consequence that the humid air in said space absorbs radiation and this lowers the efficiency. In infra-dryers of prior art, moisture transfers from the surface of the web that is being dried to the air virtually only by effect of free convection, and this lowers the evaporating power.